Multiple mode communication systems use common components for multiple functions. For example, a multiple mode communication system may use a radio communication device, such as a portable radiotelephone handset, to communicate both in a cordless mode of operation and a cellular mode of operation. In the cordless mode of operation, the handset typically communicates at low power with a cordless base station having a radio coverage area of a few hundred feet. The cordless base station may couple to one or more local loops of a public switched telecommunication network (PSTN). Thus, the handset may communicate in the cordless radio coverage area through the cordless base station with telephonic devices that couple to the PSTN.
In the cellular mode of operation, the handset typically communicates at moderate power with a cellular base station having a much larger radio coverage area relative to the cordless radio coverage area. Accordingly, the handset may be located a few miles away from the cellular base station. The cellular base station typically couples to the PSTN through a mobile telephone switching office (MTSO). In a typical cellular system, multiple cellular base stations provide coverage to a geographic area. Each of these cellular base stations overlap portions of their coverage to allow handoffs between cellular base stations to occur. Thus at any specific, confined geographic location there is one, and frequently more than one, cellular base station providing coverage for that particular location. The handset may then communicate through one of the cellular base stations with telephonic devices that couple to the PSTN.
In a typical situation, a cordless base station may be located near a user's residence or work place. Cordless mode operations are available through the handset when the user is near the user's residence or work place, and cellular mode operations are available in other locations.
The earliest multiple mode radiotelephones combined separate cordless transceivers and cellular transceivers in a common handset. The cordless and cellular transceivers operated in different frequency bands using different communication protocols. Potential interference between cordless and cellular operation was not a problem. However, the increased costs, weight, and power consumption of this dual transceiver approach made the approach impractical.
Subsequent generations of multiple mode radiotelephones use a common frequency band, or common pool of channels, and at least compatible communication protocols for cordless mode and cellular mode operations. Since a single transceiver may be used for both cordless and cellular mode operations, cost, weight, and power consumption improvements result. However, a potential for interference exists between the cordless mode and cellular mode operations because channels used for cordless operations in one location may cause interference with the same channels being used for nearby cellular operations.
One prior art multiple mode communication system is configured so that a cordless base station monitors conventional reverse cellular channels to determine which cellular channels are not in use where the cordless base station is located. Once the cordless base station finds a clear channel, it entirely consumes the channel by broadcasting a pilot signal which mimics a conventional cellular control channel. Unfortunately, the use of a continuous pilot channel prevents the pilot channel from carrying user communications which leads to an inefficient use of the frequency spectrum. Moreover, the constant transmission of pilot signals from various cordless base stations increases the background noise for all cordless and cellular mode communications taking place in the surrounding area and further increases interference.
Another prior art multiple mode communication system, such as the one described in U.S. Pat. No. 5,594,782 to Zicker et al., and entitled "Multiple Mode Personal Wireless Communication System", employs non-piloted cordless operation to reduce the likelihood of interference. In other words, the cordless base station does not constantly transmit a radiofrequency pilot signal, i.e. the cordless base station is RF-silent unless communication is first initiated by a radiotelephone. Thus, interference from the constantly transmitted pilot signal is prevented. This prior art system causes the radiotelephone to initiate communication with the cordless base station by transmitting an access message to the cordless base station when the radiotelephone determines that it is in a geographical location where communication with the cordless base station might be possible.
This prior art system causes the radiotelephone to detect the radio frequencies of the control channels transmitted by the cellular land stations at the geographic location at which the radiotelephone is currently located. The radiotelephone is then configured to compare a cellular base station identifier extracted from the detected control channel to a preprogrammed list of cellular base station identifiers, referred to herein as local station identifiers, that may be detectable within the cordless radio coverage area.
When the radiotelephone finds a match between the detected base station identifier and a local station identifier from the preprogrammed list, the radiotelephone is enabled to transmit a cordless access message to attempt to locate its corresponding cordless base station. However, when the detected station identifier matches none of the local station identifiers from the preprogrammed list, the radiotelephone will not attempt to locate its corresponding cordless base station, thus eliminating unnecessary transmissions.
In order to generate the preprogrammed list of local station identifiers, during or prior to activation of the radiotelephone, this prior art system compares a proposed geographic location of the cordless base station with computer modeled predictive propagation plots of the cellular system in order to determine the strongest cellular base stations near the proposed location. The cellular base stations which are predicted to have the strongest signals, known as better-servers, are then placed in the preprogrammed list of local station identifiers.
While this prior art system reduces the likelihood of interference by reducing the number of unnecessary transmissions, this system requires a programming process to be performed per paired unit (i.e. radiotelephone and cordless base station combination) for each specific geographic location it is to be used in. The programming process introduces additional handling and cost, and slows down system delivery time since unit pairs cannot be preprogrammed for operation and placed in stock. In addition, error is introduced in the predictive modeling and geographic location processes employed by this prior art system. Moreover, the location process employed by the radiotelephone poses problems when the operating frequencies are changed or reassigned within the cellular network, or if cells associated with a particular cellular base station are divided. When the frequencies are altered or when the cells are divided, the radiotelephone may erroneously transmit cordless network access messages and increase the number of unnecessary transmissions, or fail to transmit an access message at the appropriate location to permit subsequent cordless mode operation.
Thus, when the frequencies are altered or when the cells are divided, the frequency-dependent locating process must be correspondingly altered. This results in significant reprogramming effort and cost. In addition, there may be a programming delay between the actual frequency changes and the reprogramming of the components, during which the location process may cause the radiotelephone to erroneously transmit cordless network access messages or fail to transmit desirable access messages.
Therefore, what is needed is an automatic process for determining a list of local station identifiers, or better-server identifiers, for cellular base stations having a radio coverage area which overlies a radio coverage area of the cordless base station. Such a system should be able to update this list periodically in order to adapt to changes in the cellular frequency allocations.